is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically.
Characteristics
Strontium is a grey, silvery metal that is softer than calcium and even more reactive in water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. It burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C it will only form the oxide spontaneously at room temperature.[2]
Because of its extreme reactivity with oxygen and water, this element occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestite. It is kept under a liquid hydrocarbon such as mineral oil or kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal will ignite spontaneously in air at room temperature. Volatile strontium salts impart a crimson color to flames, and these salts are used in pyrotechnics and in the production of flares. Natural strontium is a mixture of four stable isotopes.
90Sr is a by-product of nuclear fission which is found in nuclear fallout
This isotope is one of the best long-lived high-energy beta emitters known, and is used in SNAP (Systems for Nuclear Auxiliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is required. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr. 90Sr confined inside a concave silver plaque is also used for the medical treatment of a resected pterygium
90Sr has been used as a power source for radioisotope thermoelectric generators (RTGs). 90Sr produces about 0.93 watts of heat per gram (it is lower for the form of 90Sr used in RTGs, which is strontium fluoride).[22] However, 90Sr has a lifetime approximately 3 times shorter and has a lower density than 238Pu, another RTG fuel. The main advantage of 90Sr is that it is cheaper than 238Pu and is found in nuclear waste.
http://en.wikipedia.org/wiki/Strontium
90Sr is a radioactivity hazard. 90Sr undergoes β− decay with decay energy of 0.546 MeV to an electron and the yttrium isotope 90Y, which in turn undergoes β− decay with half life of 64 hours and decay energy 2.28 MeV for beta particles to an electron and 90Zr (zirconium), which is stable.[2] Note that 90Sr/Y is almost a perfectly pure beta source; the gamma photon emission from the decay of 90Y is so weak that it can normally be ignored.
http://en.wikipedia.org/wiki/Strontium-90
Just from the Chernobyl results one can see that Sr-90 is a much larger hazard to people than Pu-238 (or Pu in general), Sr is highly absorbable by the body when ingested with respect to very insoluble Pu. This is why the US does not use Sr-90 in RTGs, and also for the fact that the half-life is too short for long duration missions. Also, alphas are better fro RTGs as they range out much quicker, and all the heat can be captured, and shielding requirements are slightly lower for alphas then betas making Pu RTGs lighter. One really wants to avoid gammas. The alphas from the transuranic elements are all around 5 MeV / decay, which gives them generally a higher power density too (0.46 W/kg(Sr) vs. 0.54 W/kg(Pu). Lower power density = lower temperature = lower efficiency. Am-241 is a decent alternative as it is also basically a pure alpha emitter, but the power density is also lower than Pu or Sr, but 5 times the half-life if you need a very long term power sources.
Just my $.02.
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